Efficiency Analysis of Solar Assisted Heat Supply Systems in Multi-Family Houses
Oliver Arnold1, Oliver Mercker1, Jan Steinweg1 and Gunter Rockendorf1
1 Institut für Solarenergieforschung Hameln (ISFH), Am Ohrberg 1, 31860 Emmerthal (Germany)
There are two major approaches to mitigate energy-related emissions of dwellings: Insulating the building’s envelope on the one hand and modernizing its heat supply system on the other hand. Often the insulation of dwellings leads to lower energy savings than expected (Greller et al., 2010), which is assumed to be related to the heat losses of the heat distribution system itself. So attention needs to be drawn to the modernization of the heat supply system, which can lead to a significant reduction of the building’s final energy demand (Jahnke et al., 2015). This paper is following this approach and focuses on the heat supply and distribution system of multi-family houses (MFH), representing a significant share of Germany’s residential market. The present work compares different systems for multi-family houses by means of its primary energy demand, energy efficiency and economic aspects. Heat distribution losses are analyzed and their relevance for efficient systems is discussed. Based on this analysis, the work highlights effective integration routes of solar thermal supply systems which may lead to increased efficiency of the overall heat supply system.
Measured energy demands of insulated dwellings are often higher than theoretically expected, as shown by Majcen et al. (2013). One reason for this are so called ‘rebound effects’ caused by the dwelling’s inhabitants, such as higher room temperatures or higher air exchange rates, which are difficult to model. Another, technical reason for the observed model/measurement divergences are commonly related to heat losses of the heat distribution system. A part of the distribution losses replaces normal operation of the room heating elements and thus may be credited to the energy demand. However, if there is no heat demand at a certain time and in an individual room, occurring distribution losses lead to overheating and as a result to increased transmission and ventilation losses of the building. In this case, the usability of the heat losses is very low. This effect is pronounced in well insulated buildings. In the present paper, this situation is analyzed by means of simulation studies.
A detailed model of a multi-family house has been established, using the TRNSYS software as appropriate modelling environment. The underlying building model refers to typical construction types and equipment which is representative for multi-family houses in Germany. The model allows the dynamic investigation of temperatures, mass flow rates and energy balances as well as the impact of heat distribution losses of individual rooms as well as the entire building. This allows a detailed investigation of the heat losses and their potential contribution to the space heating demand. As a matter of fact, such analysis must be based on simulations, because reproducible and highly analyzable field data are impossible to determine because of the enormous number of parameters, the inhabitant’s behavior, the building and weather dynamics and distributed energy flows.
The modelling of the building’s heat demand is based on an investigation of different systems that will be compared and rated in this paper. This investigation is initiated concerning the most common heating system in Germany. Based on this, different approaches are rationalized to reduce respective heat distribution losses, in order to improve their usability and – thereby – to decrease the system’s energy demand.
Eventually, a solar thermal system is suggested as effective approach to increase the energy efficiency of the building. The appropriate design of such solar thermal system is explained related to the analysis of building and energy system and economical aspects are reflected.
2.Simulation environment, building and meteorological model
The simulations are carried out employing the TRNSYS 17 modelling suite, a dynamic system simulation program. The temporal resolution is adjusted by a simulation time-step of one minute and the timescale of a typical simulation spans a one-year period. The building’s heat distribution system is modelled quite detailed to correctly simulate the dynamic behavior of the heat distribution and especially the heat losses. It regards over 100 duct sections (using Type 604) for proper spatial resolution of the heat distribution system in the building, which are capable of dynamically calculating heat losses under local thermal and flux conditions.
The model of the considered multi-family house (MFH) was built employing the module TRNBUILD. The model is based on statistical data of MFH designs in Germany. The model is comprising four floors, unheated basement and staircases and eight identical flats. Each flat contains five rooms and a corridor. The model assumes an occupation by two dwellers per flat. Every room of the building represents an individual thermal zone which is thermally interacting with adjacent rooms (zones) through walls, floors/ceilings and pipe ducts. In total, the model building consists of 52 thermal zones. The outer shell of the building is assumed to be insulated according to the corresponding German regulation EnEV (2014), always using the minimum values defined by here. For the internal room temperatures a set point of 20 °C is used. The climate model is based on Meteonorm data for the city of Zurich (Swiss), which is proven to be appropriate for central-European moderate climate situations, see Streicher et al. (2003). The resulting specific overall space heating demand of the model building is 35 kWh/(m² a) and a total heat demand is 56 kWh/(m² a). This includes load profiles for domestic hot water as described in Mercker et al. (2016a).
Four-line pipe network (4L)
A four-line heat distribution network with central heat generation, as shown in Figure 1, is a very common heat supply system for MFH in Germany, see Wolff et al. (2012). Heat for space heating (SH) and domestic hot water (DHW) is distributed via two separate pairs of pipes. The DHW is stored in a central storage at a minimal temperature of 60 °C for hygienic reasons. The heat generation system studied here represents a condensing gas boiler.
Dual-line pipe network (2L)
The second set-up considers a dual-line pipe network as shown in Figure 2. It comprises only one pair of pipes for both DHW and SH supply, which contains heating water. The DHW is then heated up on demand in decentralized heat transfer units, which are installed in every individual flat. Accordingly, the overall fluid temperature in the distribution pipes may be allowed to drop below 60 °C. The heat distribution network must meet a DHW comfort criterion of 45 °C draw temperature. To achieve this securely, the supply temperature is set to 50 °C in the simulation. The advantage of 2L over the 4L pipe network results from the lower forward line temperature, the absence of a central DHW-storage as well as the reduction of overall pipe length. Simplified integration of solar thermal heating technology is another advantage of 2L pipe networks in general. An optimized 2L system variant (2L-opt) allows a further reducing of the pipe network’s temperature level by electric backup heaters in the local heat transfer modules of the individual flats. The backup heaters ensure the desired tap water temperature. They are arranged after the DHW heat exchanger (compare Fig. 2).
Dual-line pipe network with decentralized storages
The design of this system is the same as standard 2L systems, but with decentralized buffer storages for DHW preparation. These buffers are assumed in each individual flat. The DHW is prepared on demand (when tapped) by near-by domestic hot water modules, which draw heat from the buffers for this purpose.
Dual-line pipe network with decentralized boilers
Another design approach to reduce distribution-related heat losses is the de-centralization of the heat generation using fossil fuels. In the present model this can be achieved by decentralized gas boilers installed in the individual flats. Because of the low-power level of the boilers, decentralized DHW storages (identical with those in the previous system) are necessary to grant for DHW-comfort. For this concept, the only heat distributed from the heat central to the flats is solar heat.
Dual-line pipe network with decentralized heat pumps
Another design approach regards de-centralized heat pumps for heat generation in the individual flats. The heat pumps are assumed to supply small, localized buffer storages. In contrast to the two previous model designs, the local storages, which contain heating circuit water, do supply heat for both SH and DHW, the latter via DHW modules. The model design assumes that the common heat source supplying the heat pumps is produced from geothermal resources through borehole heat exchangers. For this purpose, low-temperature geothermal heat is buffered in a central heat storage from which it is distributed to the individual heat pumps in the flats. In an alternative setting, the central heat supply may also be supported by a solar thermal system. In this case, the central storage keeps also the solar heat.
The results are presented in three separate sections: Firstly, regarding the optimization of the heat distribution network, secondly, regarding the utilization of solar heat and thirdly, regarding the alternative supply routes.
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